Premise
The threats of Pandaria have been quelled, Garrosh Hellscream slain, and a time of relative peace has befallen Azeroth. (And perhaps after an Old God/Emerald Dream/Outland Revamp expansion; I envision this content to be level 95-100.) Azeroth's heroes have returned to their respective homes but are soon hailed by their races' leaders as infernals rain from the sky upon capital cities and other sanctuaries. As all who are able heed the call of battle, an avatar of Sargeras appears overhead and tells of Azeroth's imminent doom as he nears his final act of unmaking.
Velen has a vision of an island being raised from the seafloor and informs Alliance and Horde leaders of it before departing in the once-again-operational Exodar to an unknown destination. Naaru emissaries plead the assistance of heroes to investigate this vision, sending them to the Isle of Ascendance. Upon arriving at this mysterious island, the heroes begin walking up a gargantuan staircase that appears to go well beyond the clouds. The stairs take them above the realm of Azeroth, through the Twisting Nether, and ultimately through a layer of celestial clouds. Once through the clouds, a grand structure resembling Ulduar presents itself in this heavenly plane. The Exodar is found docked to a walkway nearby. Heroes are welcomed by Ad'al who then ushers them into the structure before explaining more.
The structure is quickly recognized as Titan-built and as the heroes pass through its massive entryway, they find themselves in a court of unimaginable size. Overwhelming grandeur befalls them as they are greeted by the Titans themselves. The Titans explain that Sargeras means to focus the Burning Legion's efforts on Azeroth and due to the past strength and valor of the heroes, they have been summoned to invade Argus where Sargeras' throne resides and defeat the dark Titan. The heroes are directed to the Exodar where Velen, Anduinn Wrynn, and the Titan Aggramar recruit you for the Army of Light before departing for Argus aboard the Exodar.
Titanic Blessings
Titanic Blessings be the major factions in this expansion. Players will choose an element of the Titans to master under the guidance of the respective Titans. Pursuing one of these will grant the player new spells and buffs.
Eonar: Life
Norgannon: Dreams and Magic
Khaz'Goroth: Earth
Golganneth: Sky and Water
Argus
Upon arriving on Argus, players will advance in a fairly linear fashion towards Sargeras' throne, fighting through increasingly fortified zones until the Burning Legion's forces are diminished enough to assault Sargeras himself.
Army of the Light
As prophecized by Velen, the Army of the Light is a collection of the races of Azeroth, banded together to fight for their realm's survival. The Army is led by Anduinn Wrynn and Agrammar, Sargeras' former lieutenant.
Forces include all of the playable races, dragons led by the former aspects, Earthen led by Magni Bronzebeard who has been reborn in stone, Naga who have been freed from the tyranny of Azshara and the corruption of the Old Gods, and many others.
Thank you for reading this! It is all speculation of course, and I welcome suggestions and constructive criticism.